Centro-asymmetric optical fibers are those fibers having an asymmetric cross-section. Typically, asymmetric optical fibers include those that have one side of the optical fiber located near the optical guiding region, or core. The non-circular cross-section of the outer surface of such a fiber may have a predetermined geometric relationship to the transverse axes of the guiding regions, so that the orientation of those axes may also be ascertained from the geometry of the outer, e.g flat, surface of the fiber. The optical guiding regions are usually formed from a doped silica, with a circular cross-section.
Optical fibers can be used as phase modulators. Such phase modulators can be used, for example, in optical interferometric sensors, all-optical frequency shifting devices, and chirp control for high-speed data communications. In particular, phase modulators are an important component in fiber optic gyroscopes. Fiber optic gyroscopes (FOGs) detect the Sagnac phase shift using a fiber loop interferometer, and optimally, should do so with high rate sensitivity and bias stability. This may require elimination or reduction of any spurious signals. Elimination of such spurious signals may be satisfied by reciprocity in the overall optical circuit, and, for example, the phase modulator component must maintain polarization and exhibit a low residual intensity modulation (RIM).
For example, tactical FOGs require a modulator with an extinction ratio (ER) of better than 25 dB and a RIM less than several tens part per million. Phase modulators currently used in such gyroscopes include those based on the acoustic-optic effect, e.g. fiber wound PZT (lead zirconate titanate) electromechanical resonators, and those based on the electro-optic (EO) effect, e.g. LiNbO3 integrated optical waveguides. Fiber wound PZT resonators are inexpensive, optically continuous since pigtailing is not required, exhibit near-zero insertion loss and maintain polarization. However, the fiber wound PZTs exhibit a limited electrical bandwidth (high Q) and are vulnerable to extreme mechanical shocks. LiNbO3 integrated optic chips are efficient and can be broad bandwidth, but are generally more costly to produce mainly due to the stringent requirements for fiber pigtail junctions that need cumbersome angled facet and fiber-tip polish, and accurate lateral and angular alignments. Other modulator types include electrically poled optical fiber glasses, optical fibers drawn from organic electro-optic material, and those based on photonic band-gap filters, but these devices are either inefficient in modulation or difficult to modulate, e.g. such modulators have a bandwidth limit due to long modulation length.
As a result, there is a need for relatively inexpensive modulators that do not require pigtailing dissimilar waveguides, and are easy to manufacture, while exhibiting substantially wide electrical bandwidth and that display residual intensity modulation less than e.g. 100 ppm over a 7 phase modulation, and/or a low insertion loss.